1. Field of the Invention
The present invention relates to optical data storage systems. In particular, the present invention is a single beam direct read during write system for use in an optical data storage system.
2. Description of the Prior Art
Optical data storage systems are becoming an increasingly popular form for storing digital information. Data storage systems of this type include an optical record carrier for storing information in digital form, and an optical head for writing data to and reading data from the optical disk. During write operations, a laser within the optical head produces a beam of radiation having a write intensity level. The beam of radiation is directed to the optical disk, and produces optically detectable changes in the reflectivity of the optical disk at individual bit positions. During read operations, a laser beam having a lower read intensity level is produced by the optical head and directed to the optical disk. A detector within the optical head produces detector signals representative of a magnitude of the radiation reflected from individual bit positions on the optical disk. The detector signals are representative of data previously written to, and stored on, the optical disk.
In optical data storage sytems of this type it is always necessary to verify the accuracy of write operations, that is, to determine whether or not data was properly written to individual bit positions of the optical disk. Errors can, for example, occur when write operations are performed on bit positions which are defective due to variables in the processes used to manufacture the optical disk. Two general verification techniques for performing this function are known within the art.
In the first prior art technique, individual bit positions are read after they have been written. The data storage system can thereby determine whether or not the information actually written to the optical disk corresponds to that which was desired to be written. This technique can be implemented with a single beam optical head which produces only one beam of radiation, or a multiple beam optical head. when implemented with the single beam optical head, the written data must be read on a second pass or revolution of the optical disk. Time is thereby wasted, detracting from overall system performance. Although a multiple beam optical head produces a second beam of radiation which can be used to read the data after is has been written by a first beam, the optics required to produce the second beam adds to the cost and complexity of the optical head.
The second prior art verification technique is known as direct read after write (DRAW), and can be implemented with a single beam optical head. Characteristics of the write detect signals are monitored immediately after the beam of radiation is turned off. These characteristics have only a small margin, as they are very short in duration. Relatively complicated circuitry is therefore needed to accurately detect these characteristics of write detect signals.
It is evident that there is a continuing need for improved verification systems and techniques for monitoring the accuracy of write operations in optical data storage systems. A system which can be implemented with a single beam optical head, and which permits the accuracy of write operations to be determined during the write operations, would be especially desirable. This system must be extremely accurate. The verificatio system should be capable of implementation with a relatively simple detector circuit so that it can be economically incorporated into optical data storage systems. To this end, the verification system must monitor characteristics of the write detect signals which have a relatively large margin. A verification system which can be easily adopted to both increasing and decreasing reflectivity-type optical disks is also desirable.